1. Field of the Invention
This invention relates to a system for temperature compensation of an optical system. It relates more particularly to a system employing a fluid interposed between a movable lens mount of an optical system and a fixed housing, wherein thermal expansion and contraction of the fluid moves the lens mount to temperature-compensate the lens system.
2. Background of the Invention
The focus of a lens system changes with ambient temperature. As is well known, an increase or decrease in ambient temperature changes the refractive properties of the lens elements, and accordingly changes the focus. By moving one or more of the lens elements toward or away from the desired focal plane in response to temperature changes, focus may be maintained. The actual required movement of the lens elements may be determined according to well-known principles.
The invention relates to a particular implementation of a thermal compensation system which may be employed to provide any desired movement-temperature characteristics once they have been determined.
Many previous temperature compensation (also referred to as athermalization) mechanisms have employed solid members such as alloys or plastics as the compensation elements. The linear expansion coefficient of the solid member and other structural features are selected such that its expansion and contraction gives the lens system the desired movement characteristics.
In these systems, however, the ability to compensate a given optical system under widely varying conditions has been limited, since a solid member is limited to expanding primarily in only one dimension.
It has also been suggested to employ a fluid of a type which expands and contracts with temperature, instead of a metal or plastic material, as one part of the compensation element. For example, U.S. Pat. No. 3,612,664 discloses an optical focus compensating device having a chamber filled with temperature-responsive fluid, and a piston which forms part of the chamber wall. The piston is spring-biased. A pivoting linkage is provided to move a lens mount in response to temperature changes in the fluid. An optional electric heat source may be provided adjacent to the fluid chamber, for varying the fluid temperature from a remote location.
U.S Pat. No. 4,525,745 discloses a projection television system wherein a focusing lens is mounted in an internal spiral groove of a housing, and means are provided for rotating the lens to move it one way or the other along the optical path and thereby change the focus. The housing has a molded internal spiral groove, into which a tab on the outer peripheral surface of the rotatable lens mount fits. One disclosed means for rotating the lens (FIG. 8) is a piston-cylinder-linkage arrangement similar to the '664 patent, mounted between the periphery of the lens and a point on the outer housing. In FIG. 6A, another piston-and-cylinder arrangement is shown. This patent discloses polyethylene glycol as a fluid material which may be employed.
These prior art systems require complicated linkages and awkward component arrangements that are disadvantageous for use, for example, in compact telescopes.
Also, the fluid reservoirs in these systems cannot be located near enough to the lens to provide compensation according to the actual lens temperature.
Accordingly, an object of the present invention is to provide a thermally compensated focusing mechanism which is simple, requires a minimum of moving parts, and is compact.
Another object is to provide a system which employs a fluid in order to obtain and take advantage of expansion in three dimensions.
A further object is to locate the fluid in greater proximity to the lens being compensated than was possible in the prior art.